This invention relates generally to the field of printing using a stencil or mask and a print medium spreader, such as a squeegee or a doctor blade. It has specific applications with printing materials, such as solder paste used with surface mounted microelectronic devices ("SMDs").
Surface mount microelectronic devices (SMDs) enable highly reliable packages with improved performance, increased functionality, and decreased size. The devices have as many as 500 leads and pitches between leads as small as 0.3 mm, forcing the industry to devise and improve certain surface mount assembly processes. Unlike placement, reflow, and many other processes, where the technology used for surface mount assembly has matched or surpassed demands, the application of solder paste via stencil printing, has remained a rudimentary procedure. Defect-free solder joints require, among other things, the application of paste with sufficient pad coverage, uniform height, and favorable slumping characteristics as a necessary condition. Such defects include bridges, solder balls, and voids.
Since the quality of solder joints is a function of the environment, numerous studies have focused on characterizing the physical behavior of paste and how its properties affect printability and solderability (J. W. Evans and J. K. Beddow, "Characterization of Particle Morphology and Rheological Behavior in Solder Paste," IEEE Transactions on Components, Hybrids, and Manufacturing Technology, 10(2), pp. 224-231, (June 1987), R. E. Trease and R. L. Dietz, "Rheology of Pastes in Thick Film Printing," Solid State Technology, 15(1), pp. 39-43, (1972) and B. Rooz-Kozel, "Solder Pastes" Surface Mount Technology, (1984)). Most of these efforts have relied on off-line experimental methods for process optimization. Although these approaches have improved certain operations, many issues remain unaddressed.
One of the major problems associated with the printing process is that paste properties vary during assembly processes. Solder paste solvents evaporate as a function of time, temperature, and humidity, causing oxidation and other unfavorable conditions. Furthermore, mechanical paste properties, such as viscosity and elasticity, change with exposure to the environment as well as with the amount of work applied to the paste, e.g. squeegee speed and duration of print. These changes to the paste adversely affect joint quality and frequently require adjustment of assembly equipment to maintain product quality and throughput at tolerable levels. The retuning procedure is time consuming, costly, and requires human expertise. Off-line methods cannot be used for closed loop control. Control actions should occur dynamically in accordance with measurements which characterize real time changes to material properties, rather than occurring on an ad-hoc run-by-run basis.
Very little research has been conducted in on-line monitoring of paste properties. One study explored the application of impedance spectroscopy (C. R. Herman, V. A. Skormin and G. R. Westby, "Application of Impedance Spectroscopy for On-line Monitoring of Solder Paste," ASME Transactions, Journal of Electronic Packaging, 115, pp. 44-54, (March 1993) and M. A. Seitz, R. W. Hirthe M. N. Amin and M. H. Polczynski, "Monitoring Solder Paste Properties Using Impedance Spectroscopy," ISHM Proceedings, pp. 503-509, (1992)). This technique measures the electrical frequency response of solder paste by applying an oscillating voltage and measuring the corresponding current. An equivalent circuit model of the paste was derived from experiments, and the values of various capacitance's and resistance's were determined and correlated to physical properties and phenomena occurring within the paste. Since electrodes are placed on a test pad on the printed circuit board, electrical impedance of solder paste can be measured throughout the entire assembly process. Hence, the data gathered from impedance spectroscopy can provide useful information in terms of creating new Statistical Process Control variables.
The physics of the various assembly processes involved in paste printing are not well understood. This has precluded attempts to couple material monitoring to real time process control according to an understanding of what actually happens.
Thus, a great need exists for an understanding of the physics related to spreading solder paste and similar printing materials. In addition, a need exists for an apparatus that makes use of the physics, once understood, to control the application of printing materials such as solder paste in a real-time, closed loop manner. Accordingly, the objects of the present invention include to develop an understanding of essential aspects of the physics of the application of printed materials such as solder paste. Another object is to monitor the application of solder paste and similar materials, and to generate measurements or indications of parameters that characterize the printed material as it is being applied. It is also desirable to consider these measured parameters, and use them to control or change the process parameters of the apparatus applying the printed material.